Frequency-shift carrier telegraph system



July 6, 1954 J. L. HYsKo ETAL FREQUENCY-SHIFT CARRIER TELEGRAPH SYSTEM Filed July 29, 195o v 3 Sheets-Sheet l A T TOR/VE Y x Yum@ $5959...

J. L. HYSKO ETAL FREQUENCY-SHIFT CARRIER TELEGRAPH SYSTEM Filed July 29, 1950 f July 6, 1954 3 Sheets-Sheet 2 AIAA mwN

ubi www By Y anmrsn Afro 1/ MAA J. L. HYSKO ET AL FREQUENCY-SHIFT CARRIER TELEGRAPH SYSTEM Filed July 29, 1950 `July 6, 1954 3 Sheets-Sheet 5 a. osrenoonr Jn. /NVEA/ropsm t A By s. f: u rsoN transmission.

Patented July 6, 1954 FREQUENCY-SHIFT CARRIER TELEGRAPH SYSTEM :lohn L. Hysko, Summit, N. J., Bernard Ostendorf,

Jr., Stamford, Conn., and Wilton T. Rea, Manhasset, and Edward F. Watson, Larchmont, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 29, 1950, 'Serial No. 176,724

13 Claims. l

This invention relates to telegraph transmission systems, and particularly to carrier telegraph. transmission systems of the frequency shift type, in which marking impulses are represented by transmission of alternatingcurrent o'f one frequency and spacing impulses are reprensented by transmission of ralternating current of another frequency.

An object of the invention is to provide automatically reversible carrier telegraph transmis- 'sion from either of two or any one of more-than 'two -telegraph stations to the other or others.

Another object of the invention is to avoid the introduction of extraneous or garbled characters during the transition from one to the other of transmitting and receiving conditions.

`Another object of the invention is to enable any receiving station to break transmission from a station that is sending.

Another object of the invention is to render stations unresponsive to transmission channel echoes of their own transmission.

The invention features la mark-'hold circuit arrangement for holding the circuit that impresses received signals on the teletypewriter in rrest condition `in response to the conditions'of outgoing transmission from that station or ab sence of incoming carrier.

The invention also features the use )of the spacing frequency to -obtain a break against The invention also Vfeatures an arrangement for temporarily locking-in a Yreceived breaks'ignal to assure mutilation ofv copy when stations begin sending substantially simultaneously.

'For a complete understanding of theinvention reference may be'had to 'the `following detailed descriptionto be interpreted in the light of the accompanying drawings inwhich:

`Fig. 1 represents in block 'diagram form 7the components of a telegraph station in accordance with a preferred embodiment of the invention; and

Figs. 2 and Y3, when placedside by side with Fig. 3 at the right of Fig. 2, show the circuits embodying the components of a telegraph station as represented diagramm'atically in Fig. l.

Referring now to the drawings and particularly to Fig. l, upon which a rbrief and 'general description of the invention will be predicated, a teletypewriter I which comprisesprinting `telegraph transmitting and receiving equipment is Voperatively connected to impress marking and spacing signals upon a sending relay 2 and vto receive marking and spacing signals from a receiving relay 3, the transmitted and received signals being impressed upon one and the same loop circuit in which teletypewriter I is connected. Sending relay `2 repeats the signals transmitted `by teletypewriter I to a signal delay circuit 4 and alsocontrols -a send-hold circuit 6 and a break delay circuit 1.

The send-hold Vcircuit 6 activates an output switch '8 which .passes alternating currentat the marking or the spacing frequency from a .frequency shift oscillator V9, and initially` passes the marking frequency, this being the frequency Y normally generated by the oscillator, through a sending band-pass filter I-I to a hybrid coil I2 through which the marking signal is impressed upon the outgoing telegraph channel I3. Thus a marking signal has been impressed Lupon transmission channel I3 although sending relay 2 is repeating a spacing signal, namely the start impulse of the Viirst code combination to 'the 'signal delay circuit 4.

The signal vdelay circuit lIl, after delaying the mark-to-space transition of the start impulse and all subsequent impulse'transitions by yabout one-half pulse length, passes them Ato a modulator Ifli winch modulates the frequency shift 'oscillator s to cause it to transmit lalternating current at the spacing and marking 'frequencies in vaccordance kwith the spacing and marking impulses repeated by the sending relay. The momentary ymarking current that is `transmitted preceding.signalftransmission is effective at stations receiving it to convert those stations'from a mark-hold condition to a receiving condition, as will presently 'be described, to render them responsive to'incoming signals. The momentarily transmitted markingcurrent prevents excessivedistortion of the firstcharacter signal following an idle period, because the line transition from, zero signal condition to spacing signal condition differs from the `transition from mark to space,fand the momentary mark-ing places the line in the Ymarking condition before the markto-space transition of the nrs-t start pulse occurs. The momentarily transmitted marking current also may exercise a controlover line or terminal amplifiers or repeaters. Such amplifiers may be pro-vided with an automatic gain control feature,- an'd since in vthe idle lcondition of the stations `the frequency shift oscillators 9 are disconnected repeat the signal impulses at normal rather than excessive amplitudes, thereby avoiding distortion in the start impulse and possibly other impulses of the nrst character.

The send-hold circuit 6, in addition to activating the output switch 8, also is connected to control the mark-hold circuit I6 to cause it to impose a marking condition on receiving relay 3 during responses of sending relay 2 to spacing signals originated by the transmitter of teletypewriter I, so that outgoing signals reaching the receiving circuit directly through hybrid coil I2, or as echoes returned over channel I3, shall not be eective upon the receiving relay as are true incoming signals, there being no necessity for response of the receiving relay to these signals because, as Will be described hereinafter, local copy of outgoing messages is produced by the receiving teletypewrter under the direct control of the teletypewrter transmitter.

Received signals, comprising the momentary preliminary transmission of marking frequency, followed by the spacing and marking frequencies representing actual character signals, enter hybrid coil I2 from line I3 and are passed to a preamplifier Il. Two filters I3 and l0 are connected to the output of preamplifier the former passing only the marking frequency and the latter passing only the spacing frequency. Both of the iilters I8 and I9 pass their frequencies to a Areceiving signal amplier 2|, where they are amplified and are then amplitude limited in a limiter 22. From the limiter 'the signals pass to a discriminator and a signal detector, designated by the reference numerals 23 and 24, respectively, and there appears at the output of the signal detector 24 a voltage of one polarity for marking impulses and of the opposite polarity for spacing impulses. The signal voltages produced by signal detector 24 are passed to an output control circuit 25 which passes them to receiving relay 3 which in turn repeats them to teletypewriter I. Output control circuit 26 is capable of passing the signals to receiving relay 3 only when permitted to do so by mark-hold circuit IS. The control path from send-hold circuit 6 to mark-hold circuit l5 becomes effective during outgoing transmission to cause the mark-hold circuit to render the output control circuit unresponsive to signal output at the signal detector 2Q. rlhe mark-hold circuit l imposes a like condition on output control circuit 26 when there is no outgoing transmission and no incoming carrier of either marking or spacing nature, and the control path from signal detector 24 to mark-hold circuit I6 indicates that there must be signal reception and detection to cause the mark-hold circuit It to permit the output control circuit 25 to pass signals to receiving relay 3.

The spacing signals passed by filter i3 are also impressed on a break amplifier 2?, the break signal being a prolonged transmission of the spacing carrier frequency. The signals amplined by the break ampliiier 2 are detected in break detector 23 and are then impressed upon break delay circuit "i, Each time that sending relay circuit 2 responds to a spacing signal it imposes on the break delay circuit 'i over a control path previously mentioned, a condition representative of the transmission of a spacing signal and the break delayl circuit prolongs this condition even after the end of the spacing signal to which sending relay 2 is responding for an interval corresponding with the echo time of the transmission system. If the break delay circuit is responding to a spacing signal received from the break detector circuit 28 when it times out the echo time in connection with the response of sending relay 2 to a spacing signal the break delay circuit i, through a control path to the output control circuit 26, causes the receiving relay circuit 3 to impose a break on the loop circuit of teletypewriter I. Upon the imposition of the break signal upon the loop circuit of teletypevvriter I the send-hold circuit 0 is caused to prolong this break condition for its own delay interval, although the received break signal may terminate. This assures that the break signal shall be impressed on the loop circuit of teletypewriter I long enough for the operator to become cognizant of it and in this way transmission is broken up and the operators become cognizant of this breakup when two begin to transmit substantially simultaneously.

With the arrangement described in the foregoing paragraphs, providing as it does for suppression of carrier transmission during idle conditions, and with the station teletypevvriters rendered responsive to message transmission by the momentary transmission of marking carrier preceding the first signal of message transmission, a plurality of stations may be associated with the transmission channel I3, any one of them may begin transmitting When the system is idle and all of the others, if actually in operation, will receive the message. Moreover, any one of the stations receiving the message may transmit a break signal which Will be received by all stations and will break transmission at the station that is then transmitting.

Referring now to Figs. 2 and 3 for detailed consideration of the circuits, and particularly to granted April 18, 1933, to S. Morton et al.

Fig. 3, the reference numeral 30| designates tlie receiving selector magnet of a teletypewriter printer and numeral 302 designates the transmitter, these elements being enclosed in dotted line rectangle 303 which represents the complete teletypewriter transmitter receiver unit. This unit may be of the type shown in Patent 1,904,164 That patent shows a keyboard transmitter mechanism in combination with a teletypewriter printer. If desired the transmitter 302 may be a tape transmitter, such a transmitter being exemplified in one form in Patent 2,055,567 granted September 29, 1936, to E. F. Watson. The disclosures of these patents are incorporated herein by reference as part of the present specification.

Selector magnet 30| and transmitter 392 have their terminalsI connected to plugs 306 and 33'! that are insertable into jacks 338 and 3559, respectively. When the plugs are inserted into the jacks an energizing circuit for selector magnet 33|, supplied by the potential difference across resistor Sil of a potential divider comprising resistors Sii, 3i2, 3I3 and 3M in series between positive and negative terminals of directcurrent power suppiy or battery, is traced from the positive terminal of resistor 3H through theV sleeve of jack 305, sleeve terminal of plug Winding of selector magnet 30|, tip of plug 33, tip contacts of jacks 333 and 309, tip of plug 3d?, break key 3%, transmitter 302, sleeve terminal of plug Sill, sleeve of jack 303, variable resistor SiS to the anode of tube 3 il and also to the negative terminal of resistor 3| The positive and negative battery or power supply connections indicated in the drawings have those pclarities relative to ground which is considered as zero assenso potential. The Vtube 3| 1 has its cathode zoonnected to ground so that parallel V.conductive paths through selectormagnet 302| asju'st traced and through resistor `3I| I join at the anode of tube 3|? and extend to groundl through that tube. As will be :described hereinaftena positive potential 'will be 'assumed to be applied :at .this time ltothe "grid of tube 3H to render that tube conductive. Positive `potential is r`also applied through variable resistor 3| 8 to 'the screen of tube 3|`|, rand the desired energizing .current for selector magnet 30| is established by adjustment of 'variable resistor 316 vand 4potential divider 3|8.

The circuit of selector magnet `335| is interruptable by transmitter Y302 in accordance 'with the spacing impulses of signals to be transmitted bythe transmitter, vand in this manner ilocal copy of transmitted message material vis made. .Wththe opening, lin this manner, `of the conductive path through selector magnet 30| inparallel with resistor 3|| the current drawn vby tube 3|`| is supplied only through resistor 3| I, thereby increasing the drop through this resistor and causing the potential of the plate of tube 3|'| to decrease to a value near zero. This reduces the `flow of current through resistors i342, 3|3 and 3H, thereby carrying the grid of the right-hand triode section of tube 3|3, which obtains its grid potential from a contactor associated with lresistor .3|3, towardsnegative and reducing or cutting off the ow of current through that tube. Thus the right-hand triode of tube `3|9 is conductive at a current value determined by the adjustment of the contacter of resistor 3|3 for the marking condition of transmitter 302 and is con- 'ductive at reduced current value oris cut yoi for the `spacing condition.

The right-hand triode of tube 3|0 is cathode load coupled through resistor 32| .to the Vgrid of the left-'hand'triode of tube 3|3 and through resistor 322 to the grid of the right-hand triode of `=tube'323. The voltage swing of the 'grids of the left-hand triode of tube '3|9 and the rightha'nd ytriode of tube 323 is toward negative in response to the spacing impulses; which is the same as;

the voltage swing ofthe grid `of the right-hand triode of tube -3'|9, rbecause of the cathode drive vfromthe right-hand cathode of tube 3| 3, it being well understood that there isno inversion :of sig- 'nalsin a cathode output circuit. Resistor 32.2, 'in combination with condenser 324 which 'is connected vbetvveen the grid' of the-right-ihand triode section of tube 323 and ground formsv .a delay Y circuit forldelaying the response of the vright-'- liand triode section .of tube .-323 vtothe mark-tofspace and spaceeto-mark transitions. There'is tno-:delay inthe response of the left-hand triode of tube 3|9 tothe signal transitions so 'the response of that triode to therst signal impulse, `which will be. presumed to be the mark-,to-spa'c'e transition .initiating the start impulse of the iirst code combination torbe transmitted, =Wi11 bedevscribed first. f

l'The left-'hand triode of tube Bls direct coupled from its anode load `.resistor 326,' through 'conductor 32T and resistor 328 to fthe grid o'f the right-hand triode section of tube 33|,the grid being returned 'to negative through resistor 329, the resistors 326, 328 and 323 forming a potential divider between positive and negative power supply or batteryV connections. n. signal inversion takes placein the left-hand-triode of tubeV 3|3 because the output is derived lfrom the vhand triode section o'f tube `3|-9 swings toward i loadicoupledthrough condenser 332 tothe grid of theleft-hand triode of that tube, which has a :grid-.leakrem'stor 333 shunted by unidirectional conducting device or varistor 334. The left-hand triode of tube 33| is the send-hold circuit'heretoforefreferred to in the description of the block diagramr drawing by the reference numeral 6. The combination of condenser 332, resistor `333 :and varistor 334 with the grid of the lefthand triode of tube 33|, cathode driven from the right-hand triode is the equivalent of a fast operate and slow release relay, the lefttriode being quickly cut off with consequent positive plate swing 'in response to a vinark-to--space transition, manifested 'by va Apositive Vswing of .both cathodes as the conductivity of the right-hand triode increases, 'and being delayedly rendered conductive in response -to a space-to-mark transition, manifested by a negative swing of both cathodesas the conductivity of the right-hand triode decreases. The reason thatthe left-hand triodeof ytube 33| is immediately out off as the cathodes swing toward positiveis that condenser 332 quickly'charges through the forward (low) resistance :of varistor 334, resulting in no flow of charging current through resistor 333, wherefore the grid of the left-hand triode of tube 33| remains at 'groundpotentiaL When, at the end of the spacing signal the cathodes of the tube swing toward negative the grid of the left-hand triode 'of tube 33| correspondingly 'swings toward negative, maintaining the left-hand triodecut oi until the ycharge on condenser 332 Ihas leaked off through resistor 333. Preferably Ythe time constant of itis .restored to conductivity only after the termination of transmission.

The left-hand triodeof tube 33| having been cut'oi, the positivevoltage swing of itsanodeis applied-over conductors 333 and 233 and through resistors 211| and 2.02 :through varistor 203 to a divider, the yother arm comprising resistor `202 oonn'ecte'd'to negative 'power supply or battery 'terminahthe driving connection froml the .anode of theleft-hand triode of tube 33| being continued from the junction of potential divider resistors 20| and 202 `through caristor203 to a iu-notion with a'noppositelyV poled varistor 203 'and thence through grid resistor 236Y to the grid et theright-hand triode section of tube 201. "The right-hand triode of tube' 23? is the output switch previously identied by the reference numeral 8 in thefdesoriptionof Fig. 1. With the left-handvtriode of tube 33| yconductive prior to the spacing signal transition of sending relay tube 3`|9Yand the send-hold and control tube 33| the voltage lapplied from thelet-hand anoder'of tube 33| through varistor 203 to the junction point with varistor 204 and thence through grid resistor 206 to the right-hand grid of tube 231 Vis such as to hold the right-hand triode of the tube completely cut off to` suppress the `trans- 'mission vof carrier current from vthe oscillator.

The oscillator circuit, identified in Fig. l by the reference numeral 9, is comprised of the tube 2|| and associated resistors, condensers and battery or power supply connections. The righthand triode section of the tube, having an anode load resistor and an unbypassed cathode resistor, operates as an amplifier. Its grid is slightly negative with respect to ground by virtue of the potential divider comprising resistors 2|2 and 2|3, which causes current to flow to bring the right-hand cathode to nearly the same positive potential relative to ground. The righthand anode of tube 2|l is resistance-capacity coupled to the grid of the left-hand triode section through condenser 2|4 and grid return resistor 2|6 which is connected to ground. The left-hand triode of tube 2|| has its load in the cathode circuit, comprising resistors 2|1 and 2|8 and the cathode is coupled through a ladder type feedback network comprising capacity series arms and resistive shunt arms to the grid of the right-hand triode section. Condensers 22|, 222 and 223 are included in the capacity arms, and shunt resistor 224 is connected from the junction of condensers 222 and 223 to ground. From the junction of condensers 22| and 222 a variable resistor 226 is connected to the left-hand cathode and right-hand anode of twin triode modulator tube 221. From these electrodes of tube 221 another variable resistor 228 is connected to the junction of resistors 23| and 232 which, with resistor 233, form a potentiometer with the free end of resistor 23| connected to the positive terminal of battery or power supply and the free terminal of resistor 233 connected to ground.

The circuit oscillates at the frequency at which the phase shift through the feedback network from the left-hand cathode to the right-hand grid of tube 2|| is 180 degrees. Variable resistor 226 is provided for adjusting the oscillator frequency to the marking signal frequency, which may, for example, be 1445 cycles per second. Variable resistor 228 provides for adjusting the spacing frequency, which may, for example, be 1275 cycles. The manner in which the modulatoi` tube 221 controls the oscillator circuit to cause either the marking or the spacing frequency to be generated will be described hereinafter.

'The left-hand triode of oscillator circuit tube 2|| is cathode coupled from the junction of cathode resistors 2 I1 and 2|8 through condenser 234 to the grid of the right-hand triode of output switch tube 201. Prior to the response of send-hold tube 33| to the spacing start impulse the right-hand triode of tube 201 is held cut off so that a marking frequency impressed through condenser 234 on the right-hand grid of tube 201 is not passed through the tube. Uponthe raising of the grid potential of the right-hand triode of tube 201 by send-hold tube 33|, to render the right-hand triode of tube 201 conductive, the marking frequency applied to the right-hand grid of tube 201 varies the anode current in that section of the tube, causing the marking frequency to be amplified and impressed on the primary of output transformer 231. The secondary of the output transformer 231 is `connected through a sending level control network to the input terminals of marking and spacing frequency band-pass filters 238 and 239, respectively. The output of the two filters 238 and 239 is connected to hybrid coil 24| which has its center tap connected to ground through a balancingnetwork 242. From hybrid coil 24| the marking signal which is being passed by the right-hand triode of tube 201 is impressed on line conductors 243. In Fig. 1 the hybrid coil is designated by reference number I2 and the line is designated I3.

Returning to consideration of tube 323, it was previously stated that signals on the cathode of the right-hand triode of tube 3|9 are impressed upon the right-hand grid of tube 323 through a delay network comprising resistor 322 and condenser 324. The right-hand grid of tube 323 is connected through a varistor 35| to the junction of resistors 352 and 353 of a potentiometer comprising resistors 352, 353 and 354 connected in series between ground and the positive terminal of battery or power supply. The same grid is connected through oppositely poled varistor 356 to the junction of resistors 351 and 358 of a potential divider comprising resistors 351, 358 and 359 connected in series between the negative terminal of battery or power supply and ground. Potentials applied from these potential dividers through these varistors to the grid of the` righthand triode section of tube 323 limit the marking and spacing voltage swings of that grid u nder the control of the right-hand cathode of tube 3|9 to insure that with the shortest pulses that the circuit is designed to transmit the potential on the right-hand grid of tube 323 reaches a fixed steady state value before another signal transition is applied to the right-hand grid of tube 323 from the right-hand cathode of tube 3 l 9.

The right-hand triode of tube 323 has its load impedance in the cathode circuit which is coupled to the grid of the left-hand triode of that tube, so that the sloping front signals passed by the delay network comprising resistor 322 and condenser 324 and voltage limited by varistors 35|' and 356 are amplied and squared up by the left-hand triode of tube 323. This triode has its load impedance in the anode circuit which is coupled through a delay network comprising resistor 36| and condenser 362 to the grid of pentode tube 363. The voltage swings of the control grid of this tube are limited by oppositely poled varistors 364 and 366 connected to the junction of resistors 358 and 359 and the junction of resistors 353 and 354, respectively. In the delay network comprising resistor 36| and condenser 362 the delay is extended to an aggregate of about one-half of a signal impulse and the signal is further amplified in tube 363 which has `its load impedance in the anode circuit which is connected through current limiting resistors 246 and 241 to the grids of the right and left-hand triodes, respectively, of modulator tube 221. The circuits of tubes 323 and 363 comprise the signal delay circuit designated by reference numeral 4 in Fig. l and the circuits of tube 221 comprise the modulator designated |4 vin Fig. 1.

It was previously stated that the grid of righthand triode of sending relay tube 3|9 swings toward negative in response to spacing signals and toward positive in response to marking signals.

Since it is assumed that the start impulse of a code combination is being transmitted, the grid of the right-hand triode section of tube 3|9 has swung toward negative, and because cathode output is derived from that section of the tube and is applied to the grid of the right-hand triode section of tube 323, the right-hand cathode of tube 3|9 swings toward negative and carries with it, after the delay interval, the right-hand grid of tube 323. Cathode output is taken from .the right-hand triode.`

9 ofi'tube 32,3 tothelefthand grid of thatptube so; that-the right-hand cathode swings toward Vnegative and carrieswith it Ythe left-hand grid of `the tube.` Anode output is .taken from theleit--hand triode of tube 3.23so that as the left-hand grid swings. toward negative the anode; current is; reduced and the anode potential swings; toward positive, this positive swing being appliedl to the control gridl of tube 36,3. This increases the conductivity of tube 3'63 and since anode outputzis derived from the tube the anode potential swings toward neg.- ative, carrying withrit grids of modulator tube 227,'. The swing is suicient to; cut ofi tube r221, this-tube being conductive for the idle and for the marking condition. The, two triode sections of tubev 22.1 have their anode-cathode,l discharge paths in series, the path being traced from positive terminal of power supply or battery on the left-hand anode, thenceto the left-hand cathode,l thence tothe right-hand, anode, and finally to the right-hand cathode and ground. As previously stated, the right-hand vanode and` lefthand cathode are connected to the contactors of the variable resistors 22o and; 228. Two paths may be traced from these contactors to positive; battery, one including the discharge path of; the left-hand triode section of tube 22'! and the other including a portion of resistor 228 and thewhole-of resistor 23 l. Also, two paths may be l" traced from these contactors to ground, one including the discharge path of the right-hand triode of tube 22,1 and the other including a portion of resistor 228 and the wholel of resistors 232 and'v 233 in series. paths ofk tube '2.2-1 are shunted across a portion of the frequency determining resistance in the oscillator circuit.

Prior to the application of the mark-to-space transition to the grids of tube 2-2 1f, the left-hand grid of the tube is connected to a potential of approximately 48 volts positive with respect to ground as determined by conduction in a varistor 248 connected to the junction of resistors 23.2

and 233 of the `potential divider comprising re- The potential applied to the right-hand grid of tube 221 prior to the sistors 23|, 232 and 233.

signal transition is such as to cause a current substantially at the saturation value-in that triode section and the same current flows in the left-hand triode section, but this current is. below the saturationvalue for the left-hand triode because that triode has a higher plate potential, since the two discharge paths are in series and the left-hand anode is. connected to the positive .jr

battery or power supply terminal. Thus negative feedback in the left-hand triode of tube 221 causes its cathode to be held very close to a poten-- tial of 50 volts positive relative to ground.

When the grids of tube 221 swing negative in responsel to4 the mark-to-s-pace transition, the swing is suiicient to-cut off both sections oi the triode. This amounts to opening of the low resistance shunt that was connected around the frequency determining resistance in the oscillatorfcircuit when the tube 2'2| was conductive and the effect ofthe opening of the shunt path is tc shift the oscillator' frequency'Y from the marking to: the spacing frequency. This frequency is passed through the output switch comprising the right-hand triode of tube- 201 and through transformer 2-3-1, filter 239 and hybrid coil 243| to the transmission line 243. Resistor 232 is very small compared to resistors 23| and 233, so that the potential at the junction of resistors 23|` and 2312 Thus the two discharge CLI tube 3.611.

is only slightly higher than` the LlS-volt limiting potential on the left-hand grid; of tube 22T, supplied through varistor 248,-4 is in fact approximately 50. volts. Thus-the oscillator tuning resistances are returned to a potential having. a value of approximately- 5'9- voltspositive with respect to ground at the junction of resistors 23| and '23 -2f only, when tubeZZ-l' is cut off, 0r to that potential at that potentiometer point and to-.a likepotential also,v at the left-hand cathode of tube 221 when that tube is conducting. Thus the cutting on and restoring of' conductivity in tube 22TH to vary the tuning` ofV the oscillator by the openingl andv closing of the shunt path around its frequency determining resistors is accomplishedwithoutsignicant variation in the potential applied to those resistors.

Uponthe occurrence of the first space-to-mark transiticn` after the start impulse, tube 221 is again rendered conductive to shift the frequency of the oscillator to the` marking value. During each marking impulse, tube- 2-2| is conductive and the oscillator circuit produces the marking frequency, and during each spacing impulse tube 22.1: is cut olf and the oscillator circuit generates the spacing frequency. This; continues until transmission ceases.

It should be noted that when the left-hand triode of send-hold tube 3.-3| had its conductivity reduced or cut off in response to the mark-tospace transition of the first start impulse, and applied a positive Voltage`A swing.V to the grid of the right-hand triode of tube 207 to pass the output off thev oscillatorv circuit toV transmission line 243, a similar positive voltage swing was applied over conductor 236 Vto the grid of the lefthand triode of tube 25|. The left-hand triode ofY tube 25|, which is normally cut 01T, is rendered conductive, to apply a negative voltage swing over conductor 2.50 and conductor 3553 to thefgridof; the right-hand triode of mark-hold This cuts oif the right-hand triode o f tube 3161 which, applies a positive voltage swing to the-cathode of the. left-hand triode of output control tube 3F58' that is sufficient.- tc cut on this triode, thereby rendering it unresponsive to signals applied to its grid. This grid is under the control of the signal receiving circuit, asv will presently Abe described, and because` the signal output, the signal input and the transmission line are interconnected through hybrid coil 24|., the receiving circuit responds to the outgoing signals and they are amplified,l detected and impressed on the lett-hand grid of. output control tube 3 68 in the samel manner as lincoming signals. With the left-hand triode of this tube held` at cut-off during outgoing transmission by the send-hold tube 33| through the left-hand triode of tube 25| and the right-hand triode of tube 357|., the control gridof receiving relay tube 3l?, which is coupled to both anodes of tube 358, is maintained in the marking, condition. The righthand triode of tube 358 is employed for registering the reception of a break signal, as will be described hereinafter.

of the final signal code combination. The lefthand triode of ,tube 33|. also. cuts offthe left.-

i i hand triode of tube 25| which in turn restores to conductivity the right-hand triode of tube 361. This triode in turn restores the left-hand triode of tube 368 but because no signals are appearing in the output of the receiving circuits, the left-hand triode of tube 368 does not become suiciently conductive to impress a spacing voltage swing on the vcontrol grid of receiving relay tube 3l1.

It will now be assumed that message transmission is initiated at a remote station, the signals being received over the line conductor 243. A iirst signal received is a momentary interval of marking carrier followed by the spacing carrier representing the start impulse of the first code signal. Received signals appear across resistor 252 included in the circuit of hybrid coil 24| and these signals are amplified in preampliner tube 253 comprising two triodes, the lefthand triode being a voltage amplifier to the anode of which the right-hand grid is coupled through gain control 254. The output ofthe right-hand triode is derived from the cathode and is impressed on band-pass filters 256l and 251 which pass the marking and spacing carrier frequencies, respectively.

The outputs of the marking and spacing iilters 256 and 251 are coupled through resistors 258 and 259, respectively, to the grid of the left-hand triode of tube 26| so that both frequencies are amplified in that triode. The output of the spacing lter 251 only is also coupled through resistor '262 to the grid of the right-hand triode section of tube 26| so that the spacing frequency only is amplified in that triode. The left-hand anode of tube 26| is coupled to the grid Vof the lefthand triode section of tube 263 for additional amplication of the marking and spacing frequencies and the right-hand anode of tube 26| is coupled to the grid of the right-hand triode section of tube 263' for additional ampliiication of the spacing frequency. The reason for separate amplication of the spacing frequency is for the purpose of detecting break signals, as will be described hereinafter. The anode of the lefthand triode of tube 263, in which both the marking and spacing frequencies are amplified, is coupled to the control grid of pentode tube 264 which is operated as an amplitude limiter.

The anode circuit of tube 264 is traced through resistor 266, conductors 265 and 365 and parallel resonant circuits 31| and 312 connected in series, 'the circuit terminating at positive battery or power supply terminal. Resonant circuits 31| and 312 together comprise a frequency discriminator, substantially all of the output voltage appearing across resonant circuit 31| when the marking carrier frequency is being applied to the control grid of limiter tube 264 and practically all of the output voltage appearing across resonant circuit 312 when the spacing carrier frequency is being applied to the control grid of limiter tube 364. Resonant circuit 31| is coupled through transformer 313 to a signal detector circuit 314 which is a full wave bridge rectifier circuit having output load resistor 316. Resonant circuit 312 across which a signal voltage is developed in response to the spacing carrier frequency is connected by transformer 311 to signal detector circuit 318 which is a full wave bridge rectifier circuit having output load resistor 316. Resistors 316 and 318 are included in a rectifier biasing circuit traced from ground through these resistors, and resistors 38| and 382 to negative battery or power supply terminals. The operation of the detector circuits 314 and 318 is such that there appears at the junction of resistors 38| and 382 a voltage that is negative relative to ground for marking signals and positive relative to ground for spacing signals. These signal voltages are applied to the grid of the left-hand triode of output control tube 368.

From conductor 365 in the output circuit of limiter tube 264 a path is traced through conductor 318, coupling or blocking condenser 383, to a voltage doubler circuit 384. This circuit is non-frequency discriminatory so that a directcurrent output voltage is maintained when limiter tube 264 is responding to either the marking or the spacing frequency of carrier. The negative terminal of the voltage doubler circuit 384 is connected to the grid of the left-hand triode of mark-hold tube 361 and this voltage is suiciently negative with respect to the cathode to cut off the left-hand triode. The cutting oif of currentjin the left-hand triode of tube 361 is accompanied by a swing of its anode toward positive, and the anode is coupled to the grid of the right-hand triode of that tube to cause the right-hand triode to conduct. The potential of the right-hand anode of tube 361 swings toward negative when that triode begins to conduct, carrying with it the cathode of the left-hand triode of output control tube 368. With the cathode at this potential, the positive potential applied to the grid from the junction of resistors 38| and 382 in response to spacing signals causes sufficient current flow and consequent negative voltage swing of the left-hand anode of tube 368 to cut off receiving relay tube 3|1 which has its control grid connected to the junction of resistors 386 and 381 which are connected in series between negative and positive battery or power supply terminals as a potential divider, the lefthand anode of tube 368 being connected to the junction of resistors 381 and 388. Tube 368 is conditioned by tube 361 to control receiving relay tube 311 in accordance with the marking and spacing impulses of received telegraph signals as long as rectiiied carrier at the output of rectier circuit 384 holds the left-hand triode of tube 361 cut 01T.

When signal reception ceases, no voltage appears across the output of rectifier circuit 384, the left-hand grid of tube 361 swings toward positive, and the left-hand triode of the tube again becomes conductive, cutting off the righthand triode. This is the same condition that was imposed on the right-hand triode of tube 361 over conductor 350 under the control of the send-hold tube 33| during outgoing transmission,

and results in a positive potential on the cathode of the left-hand triode of tube 368 such that that tube is cut off. With the left-hand triode of tube 368 cut oli the control grid of receiving relay tube 3I1 receives a potential on its control grid corresponding to a marking signal and thus holds the loop circuit including teletypewriter selector magnet 36| energized in the marking condition. The mark-hold tube 361 thus imposes a marking condition onthe left-hand triode of relay 368 during intervals of outgoing transmission and during absence of received carrier.

It will now be assumed that while outgoing transmission is in progress from the station shown in Figs. 2 and 3, a break signal, comprising an interval of spacing carrier longer than ordinary spacing signal impulses, is received from a remote station. Amplified carrier of spacing frequency appears at the anode of the right-hand assenso Y triode ot-.amplier tube 263;. as previously` stated',

andlthisi anode:` isY coupled through condenser 2-.11lrandl conductors 211i and.310 to the gridy of breahdetector tubeazl.. The spacing. carrier is amplified intu'be 351i, network 38.5 in the anode circuit:- providing additional. discrimination against theimarking, frequency, and is.- rectified in half-Wave rectiercircuitv 392, Whichis coupledzta the .anode circuit. .of tube 395|. through blocking or` coupling; condenser 393,' to produce at the output of the: rectifier circuit a. positive voltage.. This; voltage is applied.; over conductors- 3.9.ltandV 29I1 andfthrough varistor-212 to the grid ofthe right-hand triodeof. tube12'13. This-.triode is part. of a break delay circuit, identified by referencev numeral 1 inzliig` .1, and which will now be: described.

Tha receiving. circuit must@ be capable of responding tozincomilng spacing; break signals. when transmission is in progress. In order to prevent falsebreaks. dueto echoes; of outgoing spacing signalait isinecessary to. introduce delay into the break patnso. as. to delay the registration of a breaksignal until echoes of; ordinary spacing signailsvha-ve been dissipated.

During outgoing transmission the mark-tospaea and. space'fto-mark transitions produced at the; cathodese of tube 331'; are applied, over conductors 3.8@ and 28,0 and; through resistors 215 and-.Zfuland varistor 216`to the gridY of the righttriode of? tube 2.5i, and. these transitions appear Vat the right-hand cathode of tube. 251-. as. a positive voltage swing for the spacing impulsesand negative voltage swing for the marking: impulses.. These voltage swings are applied to: thecathode of. the left-hand triode section of tube 211:1... This-cathode is. coupled to its grid throuif condenser 218 and the grid is returned to ground through, variable resistor 219 and also through. the diode-connected left-hand triode section of tube 201.-. The arrangement is like thatof the send-hold circuit, in that condenser 2:18: correspondsv to condenser 332, variable resistor 219i corresponds to resistor 333 and the diodefconnected left-hand, triode of tube 2811l` corresponds to varistor 334.V A positive voltage swing, applied from the right-hand cathode of tube 25.I to the left-hand cathode of tube 211 in response to a. markfto-space transition of. outgoing signals cuts on? the left-hand triode of tube 211 and it remains cut on" for the duration of. thespacing signal and for an interval thereafter of. sufficient duration to cover the longest spacing signal echo, the amount of delay being controlled byY variable resistor 219. The anode off the left-hand triode of tube 211 is coupled to the; grid of the right-hand triode and with the cutting off of the left-hand triode the anode thereof. swings toward positive, applying a voltage. swing in the positive direction to the grid ofthefright-hand triode and increasing the current flow through the right-hand triode of tube 2.11'.

'Iheright-hand anode of tube 21.11' swings toward negative as the conductivity of the triode increases andv this voltage swing is applied over conductors 26u and 36e to the grid ofv the righthand. triodesection of output control tube 368. This isV for the purpose. of preventing the righthand. triode; of tube 36.8 frombeing rendered conductive. y

anode swings toward negative, and.' since. the grid vof the left-hand triodeeorthat tube is.v` con pled toA the. right-hand anode, current in the normally conductive left-hand triode-is:v cntcoi andfits anode swings positive'. This anode. is connected over conductorsV 2.60 and. 360x to the right-hand gridof'tube 36.8 but the right-*hand triode of that' tube. is held.- cut oi or: at `lors conductivity wbecause the right-hand. anode` oi tube A211 has swung toward. negative.

When the left-hand triode of tube. 2H times out the break delay interval, the .left-handtas ode becomes conducting. andcuts off the rightihandtriode. Under the assumed conditonfthat the left-.hand triode. of tube; 213 remains cut offbecause the received spacing. frequencyv which has produced an output voltage at recter SBZ-is infact a break signal,- .the grid of. the right-hand triode of tube 368 willavbe swung towardqpositive to. causethe tube to conductand reduce the Apotential of the-control gridof receiving .relay'tube 3111', thereby. registering. a. spacing signal. in teles typewriter selector magnet 39h In this Way-,xa received spacingV frequencyv which conthiuesfafter the timing out ofthe break delay circuit. imposes*v abreak on. they teletypewriter loop.`

With the left-.hand triode of tube 213 andthe left-hand. triode of tube. 2,11. cut ofi and. imi-.- pressing a positive potential` on. the right-hand gridf of tube. 368 to register theabrealc signaliin the teletypewriter loop,. the` positive. potential is. also applied through; varistor 2.81` to; the. rightehand grid ofzitube. 21ste: maintain theA right-hand; tri-ode conductive even.. though the. break signal' may have terminated with` consequent removal of thev positive potential; appl-led through. varistor 212-. With the. .right-hand.; triode of tube; 21%u remaining conductive the left-hand triode;` remains. cut-off and this triode hasthus locked itself. in the: cut-0H` condition. maintaining the spacing. signal .applied tor the; teletypewriterl loop under the control of. the rightfhand triode. of4 tube 368.

With receiving relay tube 3H' cutoff thereL is f very little voltage drop. across resistor'. 3H,

whether the'loop. circuitv is open or closedat the transmitting contacts.. 3012', because the. totaliresistance of resistors 312,313; and. 3M isL very high compared to that of resistor 31HM causes a positive voltage swing of the: right-hand.. of tube 319` to cause a marking signal transmisesion condition in the transmitting circuit while a received spacing signal is impressed on the teletypewriter loop, whetherv or not transmitter 30.2 is being operated. In. this way outgoing transmission is interrupted by the response of the receiving teletypewriter loop. to a break signal. The steady marking signal condition imposed on the left-handl triode of tube 3|9 is repeated. by the left-hand' triode of that tube, and through. the right-hand triode of tube 33| tov the leftr hand triode of that tube and to the right-hand triode of tube 251. The right-hand? triode of tube 25| isy thus held in the marking condition and the initiation of another timing-out cycle of'l the break delay circuit comprising the lefthand ytriodeof tube- 211 is'- prevented; In addi-- tion, the left-hand triode of tube 331- Wh-ich is the send-hold circuit, is` permitted to time out andrestore to its condition of' no outgoing signal transmission. The timing out interval is at least as long as the longest spacing; signal-oc.;- curring in ordinary message transmission, and: mayf be asv long asl the interval of several; char.- acter signals in successionand in any event,y

should be long enough to lndicate the fact that the distant station is trying to break in. At the end vof the timing out interval the left-hand triode of tube 33| becomes conductive and the voltage of its anode swings negative, cutting off the right-hand triode of tube 291 to suppress the transmission of carrier and also cutting off the left-hand triode of tube The left-hand anode of tube 25| swings positive and applies a positive potential through varistor 282 to cause the right-hand triode of tube 25| to conduct. `This condition of the right-hand triode of tube teletypewriter selector magnet 38|. The negative voltage swing applied to the right-hand grid of tube 213 effects an unlocking of that tube, the right-hand triode being cut off and the lefthand triode restored to conductivity.

The response of a station which is in the receiving condition to a break signal is different. When a station receives any spacing signal, that signal, in addition to being amplied and detected along with the marking signals in the manner hereinbefore described, is also amplied by tube 39| and is rectied by rectier circuit 392 to impress a positive potential through varistor 212 on the right-hand grid of tube 213. The right-hand triode of tube 213 in turn cuts off the left-hand triode but this does not lock the tube 213 in this condition by applying a positive potential through varistor 28| to the right-hand grid of the tube because the left-hand triode of tube 25| is held cut off by the send-hold tube 33 I, the right-hand triode of tube 25| is held conductive by a positive potential applied through varistor 282 to the right-hand grid, a corresponding positive potential supplied from the cathode of the right-hand triode of tube 25| to the cathode of the left-hand triode of tube 211 holds that triode cut off and the positive potential at the left-hand anode of tube 211 holds the right-hand triode of that tube conductive. With the right-hand triode of tube 211 conductive tube 213 cannot become locked up with the right-hand triode conductive and the left-hand triode cut off, nor can the right-hand triode or" output control tube 368 be rendered conductive although the left-hand triode of tube 213 is cut oi. The break signal will merely be repeated by the left-hand triode of tube 368 to receiving relay tube SE1 the same as any other received spacing signal and at the end of such break signal the right-hand triode of tube 213 will be cut oi and the left-hand triode will be rendered conductive.

The break delay circuit and associated break signal circuits provide for complete interruption of transmission in the event that two stations begin transmitting substantially simultaneously. This results from the fact that the break delay circuit is not required to time out over a period sufficiently long to clearly distinguish a break signal from an ordinary received spacing signal, but is required to time out only over an interval to accommodate the longest probable echo of outgoing spacing signals. Aspreviously set forth, a break signal is registered upon the right-hand grid of tube 358 only under the condition that both the left-hand triode of tube 213 and the right-hand triode of tube 211 are cut off. The latter triode cuts ofi only after the break delay time-out following restoration of the negative potential on the left-hand cathode of tube 211 by a reduction in conductivity of the right-hand triode of tube 25| as conductivity is reduced in that triode due to response of the right-hand triode of tube 33| to an outgoing marking signal. During a steady marking interval comprising a succession of marking elements of an outgoing character signal such as letters (elements one, two, three, four, ve and stop all of marking nature) or V (elements two, three, four, ve and stop all of marking nature) the break delay circuit may nearly or completely time out and restore conductivity in the left-hand triode of tube 218, thereby cutting olf the right-hand triode of tube 211. If a spacing signal, even of short duration, should be received under this condition and before the next mark-to-space transition of outgoing signals, it will be amplified in tube 39|, rectified in rectiiier circuit 392, and applied as a positive potential on the grid of the right-hand triode of tube 213, rendering that triode conductive and cutting off the left-hand triode whereby a positive voltage swing would be applied to the righthand grid of output control tube 358 immediately or as soon as the break delay circuit had completed its time out, as the case might be, to impress the received spacing signal on the teletypewriter loop and to lock in that spacing signal by virtue of the locking potential applied to the right-hand grid of tube 213 through varistor 28|. The received and locked in spacing signal applied to the teletypewriter loop would prevent the re- @Ording of characters corresponding to characters of attempted transmission at the `transmitter 302 and the operator, noting failure to produce home copy of those signals, would know that a break or other spacing signal had been received and locked in, thereby at once suspending outgoing transmission other than a steady marking carrier which would continue until the send-hold circuit had timed out, restored the teletypewriter loop to marking condition and cut off outgoing marking carrier. Similar interruption of outgoing transmission might possibly occur at the other station at which transmission had been started, whereupon message transmission may be restarted from the beginning from one of the stations in accordance with preestablished rules concerning order of precedence among stations where two have attempted to transmit simultaneously. The important point is that a prompt and complete breaking up of simultaneous transmission shall occur, so that each of the two stations shall not continue to transmit for an extended period of time without knowledge that the other is also transmitting. This is accomplished by the break signal responsive circuits as herein described.

Although a particular embodiment of the invention has been shown in the drawings and described in the foregoing specification, the invention is not limited to such embodiment but is capable of modification, rearrangement and substitution of parts and elements without departing from the spirit of the invention.

What is claimed is:

1. In telegraph system including a transmission channel normally having no current flowing gesagisa therein, f intelligence-signal? transmission `means o intelligencesignal transmission currents.

2. In a telg'raph system'includinga transmission channel normally having no current nowing therein, intelligence signal transmission means for'impressingmarking and spacing signal currents having different frequencies upon said channel, and means eifective as an automaticallyproduced result of initiationofoperation of said intelligence signal transmission means and. concomitantly with said operation for impressing on said channel ahead of the'rst intelligence signal transition a preliminary interval of marking frequency current so thatxall of the intelligence signal transitions including the `first shall be from one to another of4 said frequencies.

3. In a telegraph system including a transmission channel normally having no current owing therein, intelligence signal generating means for producing marking and spacing conditions of intelligence signals, a source of signaling current to be impressed on said channel, means for modulating said source in accordance With the marking and spacing conditions of intelligence signals, means eiTective upon initiation of operation of said intelligence signal generating means for transmitting signaling current from said source to said channel, and delay means intervening said intelligence signal generating means and said modulating means for providing an interval of preliminary transmission from said signaling current source before modulation thereof by said intelligence signals.

4. In a telegraph station, telegraph signal generating means, relay means controlled by said signal generating means for causing outgoing transmission of generated telegraph signals, a teletypewriter printer responsive to said telegraph signal generating means, other relay means for operating said teletypewriter printer in response to incoming signals, and holding means controlled by the iirst-mentioned relay means for suppressing operation of said other relay means in response to outgoing signal transmission.

5. In a telegraph station, a carrier current signal transmitting circuit, a telegraph transmission channel coupled to said transmitting circuit, a carrier current signal receiving circuit coupled to said transmitting circuit and said channel so as to respond'to outgoing and incoming carrierV current signals, a teletypewriter printer, relay means operable by said receiving circuit for operating said printer, and holding means controlled by'said transmitting circuit for precluding operationv of said relay means during operation of said transmitting circuit.

6. In a telegraph station, a signal receiving circuit responsive to outgoing signal transmission and incoming signal transmission, a teletype- Writer printer, signal responsive relay means for operating said printer in accordance with `the marking and spacing character of signals, holding means for disabling said relay means upon the occurrence of outgoing signal transmission, and break signal detecting means operable upon response of said receiving circuit to a spacing signal exceeding a predetermined duration for rendering'said rrelay means 'and 'consequently said printer responsive to said spacing signal despite outgoing transmission.

7. In a telegraph station, a signal receiving circuit responsive to outgoing signal transmission and incoming signal transmission, a teletypewriter printen, signal responsive relay means Y for operating saidprinter in accordance With the marking and spacing character of signals, holding meansfor disabling said relay means upon the occurrence of outgoing transmission and for a predetermined interval after the end of any spacing signal of outgoing transmission, and break signal detecting means operable upon re- -f sponse of said receiving circuit to a spacing signal continuing or occurring after the end of said interval for rendering said relay means and consequently said printer responsive to said spacing signal despite outgoing transmission.

8. In a telegraph station, a signal receiving circuit responsive to outgoing signal transmission and incoming signal transmission, signal generating means for effecting outgoing transmission, a teletypewriter printer, signal responsive relay means for operating said printer in accordance With the marking and spacing character of signals, holding means for disabling said relay means upon theoccurrence of outgoing .transmission and for a predetermined interval after the end of any spacingV signal of outgoing transmission, and break signal detecting means operable upon response of said receiving circuit to a spacing signal continuing or occurring after the end of said interval for rendering said `relay means and consequently said printer responsive to said spacing signal 'despite continued operation of said signal generating means.

9. In a telegraph system, a station having a signal receiving circuit responsive to outgoing signal transmission and incoming signal transmission, a transmission channel, signal `generating means for effecting outgoing transmission, a teletypewriter printer, signal responsive relay means for operating said printer in accordance with the marking and spacing character of signals, holding means for disabling said relay means upon the occurrence of outgoing transmission for an interval after the end of any spacing signal at least as long as the echo time of said channel to-outgoing spacing signals, and break signal detecting means operable upon response of said receiving circuit to a spacing signal continuing or occurring after the end of said interval for rendering said relay means and consequently said printerresponsive to said spacing signal despite continued operation of said signal generating means.

10. In a telegraph station, a signal receiving' circuit responsive to outgoing signal transmission and incoming signal transmission,v a teletypewriter printer, signal responsive relay means .for operating said printer in accordance with the marking and spacing character of signals, holding means for disabling said relayy means upon the occurrence of outgoing signal transmission, break signal detecting means operable upon response of said receiving circuit to a spacing signal exceeding a predetermined duration for rendering said relay means and consequently said printer responsive to said spacing signal despite outgoing transmission, and locking means for maintaining said relay means in the spacing signal condition for a predetermined interval in- 19 dependently of the duration of the received spacing signal.

ll. ln a telegraph station, a signal receiving circuit responsive to outgoing signal transmission and incoming signal transmission, signal generating means for eiiecting outgoing transmission, a teletypewriter printer, signal responsive relay means for operating said printer in accordance with the marking and spacing character of signals, holding means for disabling said.

relay means upon the occurrence of outgoing transmission and for a predetermined interval after the end of any spacing signal oi outgoing transmission, break signal detecting means operable upon response of said receiving circuit to a spacing signal continuing or occurring after the end of said interval for rendering said relay means and consequently said printer responsive to said spacing signal despite continued operation of said signal generating means, and means controlled by said relay means upon response to said spacing signal for eiecting outgoing transmission of a steady marking signal despite continued operation of said signal generating means.

l2. In a telegraph station, a signal receiving circuit responsive to outgoing signal transmission and incoming signal transmission comprising marking and spacing conditions, a teletypewriter printer, signal responsive relay means for operating said printer in accordance with the marking signal detecting and spacing character'of signals, signal detecting means for impressing different operating voltage conditions on said relay means for said marking and spacing signalconditions, non-discriminatory means for impressing an enabling voltage condition on said relay means in response vto both said marking and spacing signal conditions, and still other signal detecting means having discriminatory attributes for producing a break signal registering voltage condition in response to a spacing signal condition only.

13. In a telegraph system including a transmission channel normally having no current flowing therein, intelligence signal transmission means for impressing marking and spacing currents having different characteristics upon said channel, and means responsive to initiation of intelligence signals for impressing on said channel a preliminary current having a diiierent characteristic from that of the first current of said intelligence signal transmission.

References Cited in the file 0f this patent UNITED STATES PATENTS Number 

